Because of recent emergence of a laser having an intense output and good coherence, it has become possible to obtain a light having a wavelength of 1/2 as the second harmonic generation, hereinafter referred to as SHG, by using nonlinear optical devices.
The nonlinear optical devices are required to be crystals of relatively large optical classes of not less than 1 mm. For instance, a single crystal of KTiOPO.sub.4 which is a nonlinear optical crystal is known as a typical example of the nonlinear optical devices. The single crystal of KTiOPO.sub.4 is produced by hydrothermal methods or flux methods to be described later.
As the flux methods among these methods of producing single crystals of KTiOPO.sub.4, a method of melting and synthesizing, for example, a TiO.sub.2 solution into K.sub.2 P.sub.2 O.sub.7 and K.sub.3 PO.sub.4, (see L. Ouvard, Comptes Rendus 111, p. 177-179 (1890)), and a method of producing powders of KTiOPO.sub.4 by heating a predetermined ratio of KPO.sub.3 /TiO.sub.2 (=1 to 4) at temperatures of 900.degree. C. to 1200.degree. C. and then cooling at a rate of 80.degree. C. per hour, (see R. Masse et al., Bull. Soc. Mineral Crystallogr. 94, p.437-439 (1971)) have been known for long.
As a recent example, the following method is disclosed in the U.S. Pat No. 4,231,838. In this disclosure, KTiOXO.sub.4 and a flux K/X/O (wherein X is P or As) or precursors of these are used as ingredients. The ratio of the starting materials lies within the region of the ternary phase diagram K.sub.2 O/X.sub.2 O.sub.5 /(TiO.sub.2).sub.2 where the object product KTiOXO.sub.4 of Pna2.sub.1 type is the only stable solid phase in equilibrium with the molten flux. With the above-mentioned materials, a single crystal of KTiOXO.sub.4 large enough to be used as a nonlinear optical material is produced by the temperature gradient process, or the uniform heating process.
In addition, results of measurement of solubility curves of the single crystal of KTiOPO.sub.4 produced when K.sub.4 P.sub.2 O.sub.7, K.sub.8 P.sub.6 O.sub.19 and K.sub.15 P.sub.13 O.sub.40 are used as fluxes, and comparison with a case of using a flux K.sub.6 P.sub.4 O.sub.13 in the technique described in the above-mentioned U.S. Pat. No. 4,231,838 are reported in G.M. Loiacono et al., J Crystal Growth 104, p. 389-391 (1990)).
Thus, while tile re have been various kinds of literature on methods of producing a single crystal of KTiOPO.sub.4, none of the proposed methods can produce a crystal of single domain.
On the contrary, in crystal growth of KTiOPO.sub.4 attempted by the present inventors by using the above-mentioned K.sub.6 P.sub.4 O.sub.13, it was confirmed that the resultant crystals exhibited polarization of multi-domain.
The presence of multi-domain becomes a cause of a reduction in SHG output and deterioration of efficiency of the nonlinear optical materials as SHG, (see J.D. Bierlein et al. , Appl. Phis. Lett. 51, p. 1322 (1987)).
In order to solve this problem, a method of producing single domain by cutting out a crystal ingot. having multi-domain in a bulk shape or i n a substrate shape, and then implementing heat processing and applying electric fields during the heat processing i s employed.
For example, according to G.M. Loiacono et al. who examined the presence of multi-domain, single domain is produced by cutting out a plate from a grown single crystal of multi-domain in a vertical direction toward a C axis (polarization axis), forming an electrode on both sides of the plate (C plate), and applying predetermined voltage in a state of being heated to approximately 500.degree. C.
However, these conventional methods have the following problems: that it is difficult to produce single domain, depending on the arrangement of multi-domain (e.g., an arrangement of zigzag polarization proceeding); that the poling process including formation of electrodes is rough; and that defects generated on boundaries of the domain tend to remain even after the poling. Accordingly, it is difficult to attain sufficient production of single domain.
Thus, it is the object of the present invention to provide a method of producing a single crystal of KTiOPO.sub.4 whereby sufficient production of single domain and crystal growth of high quality can be achieved.